Endogenous prostaglandins may be involved in modulating coronary ischemia by their effects on platelet aggregation and by their direct vasoactive properties. An agent that selectively inhibits platelet aggregation without affecting prostaglandin-mediated coronary vasodilation might reduce ischemia. Sulfinpyrazone is known to inhibit platelet function and has been shown to be beneficial clinically in survivors of myocardial infarction. We therefore examined whether sulfinpyrazone would selectively inhibit platelet aggregation without altering the coronary vasodilatory response to the intracoronary infusion of the prostaglandin precursor arachidonic acid in dogs. Results showed that sulfinpyrazone inhibits both platelet aggregation and the coronary vasodilatory response to arachidonic acid in a dose-dependent manner. Moreover, the sensitivities of these two systems to sulfinpyrazone inhibition appears to be similar.